Selective enclathration of picoline isomers by a resorcinarene host

The resorcinarene host 1(4),1(6),5(4),5(6)-tetrahydroxy-2,4,6,8-tetrapentyl-3(4),3(6),7(4),7(6)-tetra(p-toluenesulfonyl-oxy)-1,3,5,7(1,3)-tetrabenzenacyclooctaphane forms inclusion compounds with 2-, 3- and 4-picoline. The structures of these inclusion complexes have been elucidated and compared, with some being found to additionally include water molecules as guests. Competition experiments were carried out to determine whether this host would selectively enclathrate any of the picolines and thus investigate the capability of this host for the separation of picoline isomers. A very interesting result was obtained for the 3- picoline versus 4-picoline competition experiment, which displays a concentration dependent selectivity, but instead of selective inclusion of the guest of higher concentration, it was in fact the guest of lower concentration in the solution which was preferentially enclathrated. This unusual result was explained in terms of solubilities of the clathrates in the two guests.